But if the root partition is greater than 50% full, then the [boot,root,swap] order will be better.

I'm thinking of a fast recovery scheme where the root partition is small and I will do an image for the disk to the end of the root partition. Having the swap partition at the end will make the image smaller and faster.

If your backup is based on the contents of the filesystem, its size will not be affected by where you place the partitions on the underlying drive. If your backup is based on a dump of the drive, then you are copying free space.